Non-volatile memory devices such as magnetic random access memory (MRAM) devices are of interest for replacement of volatile memory devices such as dynamic random access memory (DRAM) devices. Such MRAM devices include an array of individual MRAM cells which may be tunnelling magnetoresistance memory (TMR) cells, colossal magnetoresistance memory cells (CMR) or giant magnetoresistance memory (GMR) cells.
In general, the MRAM cells include a data layer and a reference layer. The data layer is composed of a magnetic material and during a write operation the magnetisation of the data layer can be switched between two opposing states by an applied magnetic field and thus binary information can be stored. The reference layer often is composed of a magnetic material in which the magnetisation is pinned so that the magnetic field that is applied to the data layer and in part penetrates the reference layer, is of insufficient strength to switch the magnetisation in the reference layer.
For example in a TMR cell the data layer and the reference layer are separated by a thin dielectric layer which is arranged so that a tunnelling junction is formed. Any material comprises two types of electrons which have spin-up and spin down polarity. In the case of a ferromagnetic layer that has a magnetization, more electron spins have one orientation compared with the other one which gives rise to the magnetization. The electrical resistance through the layers is dependent on the relative orientations of the magnetizations in the data and reference layers. This is the tunneling magneto-resistance (TMR) effect and the state of the data layer can be read by measuring the apparent electric resistance across the layers.
The data layer comprises a low coercivity material that can be switched in its magnetic direction by a megnetic field generated by column and row data-write current.
The reference layer usually is fabricated with a high coercitivity material and is permanently magnetized in a set direction during an annealing process step. In one version of the memory cell, namely the “spin-valve”, the reference layer is “pinned” by exchange coupling by an adjacent antiferromagnetic layer. In such a spin-valve, the orientation of the magnetization of the pinned reference layer remains substantially fixed.
In an alternative design the reference layer is soft-magnetic and has a lower coercivity so that the reference layer can be switched together with the data layer. In this case the magnetic field of a control current is used to switch the magnetization of the reference layer to the reference state after the data layer is switched. The coercivity of the reference layer and the magnitude of the control current need to be chosen so that switching the reference layer does not affect the data layer. In order to make switching of the reference layer easier and to reduce the magnitude of control currents required for switching the reference layer, it is of advantage that the coercivity of the soft reference layer is as low as possible. However, reference layers with low coercivities are difficult to fabricate. Hence, there is a need for a magnetic memory device in which switching of the soft reference layer is facilitated.